Nitrogen exists in nature in both the inorganic and organic form, and is cyclically converted by means of microbes and by chemical redox processes. The organic nitrogen exists in both solid and dissolved forms, as well as in the gas phase, while the inorganic nitrogen exists almost exclusively in the dissolved form or in the gas phase.
In raw water, i.e., untreated water from ground waters or surface water supplies, nitrogen in the form of nitrites is of the greatest interest from the point of view of toxicity. The reason for this is that nitrites block the respiratory mechanism in humans, and therefore any excessive intake leads to choking symptoms, which are particularly critical for infants (i.e., blue babies). Furthermore, nitrates can yield the same problem, since nitrates may easily be converted into nitrites by means of microbes, or by purely chemical reactions, in the human intestinal tract. It is thus important that both nitrates and nitrites be removed from drinking water before it is supplied to the consumer.
Among the types of purification processes which are presently used for removing nitrogen compounds are the following:
(1) ion exchange; PA1 (2) reversed osmosis; and PA1 (3) biological denitrification.
The first two of these processes, however, in most cases require high investment and operating costs, and are therefore less attractive than the biological method. Moreover, ion exchange is not usable in all countries, since toxic components are released from the ion exchange resin. According to the biological process, nitrites and nitrates are converted into nitrogen by means of microbes, and the nitrogen can then be removed, such as by aeration, for example.
Biological denitrification takes place by means of heterotrophic, facultatively anaerobic bacteria. The term "heterotrophic" means that the bacteria require a source of organic carbon as an electron donor and for the synthesis of their own carbon compounds. This carbon source, which may consist of low molecular weight sugars, simple alcohols or organic acids, is thus converted into carbon dioxide by means of oxidation. The term "facultatively anaerobic" means that the bacteria utilize molecular oxygen as an electron acceptor if oxygen is available. However, in the absence of oxygen, the bacteria can then utilize nitrates or nitrites as an electron acceptor. Thus, denitrification processes generally require that the reaction take place in a substantially oxygen-free environment. Not all of the nitrogen will be converted into molecular nitrogen, but a portion thereof will be used for the preparation of organic nitrogen compounds which are necessary for the bacteria themselves.
The bacteria also require phosphates and trace elements for their growth and propagation. Such trace elements are normally present in sufficient amounts for this purpose in the types of water which are to be purified, while phosphates must be added thereto in some cases.
The bacteria active in such denitrification processes usually belong to the genera Pseudomonas, Bacillus and Achromobacter. Normally the biological denitrification process results in an almost complete removal of nitrates and nitrites. The disadvantage in this process is that these two components are not completely converted into molecular nitrogen, and therefore cannot be completely removed from the system. In addition to the formation of organic nitrogen compounds, it has thus been shown that a bacterial dissimilatory denitrification takes place, leading to the formation of ammonium ions (see Smith, De Laune and Patrick, Soil. Sci. Soc. Am. J., Vol. 46, 1982, pp. 748-750). Ammonium ions may be formed assimilatorily, i.e., the ammonium is absorbed within the cell mass of the bacteria, as well as dissimilatorily, i.e., the ammonium is emitted to the surrounding solution during the metabolism of the bacteria. The organically bound nitrogen will also be gradually emitted to the surrounding solution, normally in the form of ammonium ions, when the bacterium dies and the cell thereof lyses.
In the case of drinking water, the ammonium may be oxidized to nitrites and nitrates before the water has reached the consumer. Thus, the biological denitrification process has not produced the desired result if the ammonium content is too high.
An object of the present invention is to supplement the biological denitrification process with a process step for removing ammonium therefrom. In this manner, all of the nitrogen components can be removed so that they cannot give rise to toxic problems in the drinking water.